What is a Preamp?
A preamplifier (preamp or just “pre” for short) is an electronic component used to condition signals from a microphone or pickup. The output signal from a microphone is generally weak and noisy. If you plugged a microphone directly into an audio interface, the sound would come out distorted and garbled. Not exactly the result you want from your audio system. That’s where preamps come in.
Preamplifiers boost the strength of a signal without significantly degrading its signal-to-noise ratio (SNR). This means that preamps make the signal stronger while doing their best to not strengthen noise as well. A signal that passes through a preamp experiences a voltage gain. For example, a 10 mV signal that enters a preamp may exit as a 1 V signal.
Designers of preamps keep the input and output impedances of their devices as high as they can (meaning the preamp is sensitive to small current inputs and doesn’t change voltage outputs with changes in current draw.) Preamps often amplify signals up to line level, the specified strength for transmitting signals between devices. If it helps, you can think of a preamplifier as a system that “fixes” audio signals so other devices can understand and use them.
Do I Need a Preamp?
The answer to this question is: it depends. Many microphones and audio interfaces have built-in preamps. These built-in preamps are enough for most basic applications. However, if you want a higher sound quality with more gain and less noise, an external preamp is what you need. If the device you are using does not have a preamp, you will definitely want to add a preamp to condition the output signal.
Even if your device has a built in preamp, you may want to add one externally. Some musicians add preamps to create vintage sounds in their recording process. Such preamps can add a ‘dirty’ or ‘gritty’ flavor to the sound.
Be careful not to confuse power amplifiers with preamplifiers. A power amplifier adds voltage to a signal so it can be output through a loudspeaker. Preamplifiers condition a signal fresh out of a microphone or pickup to be stronger and cleaner so that it can be used by other audio devices. Thus, it is a PRE-amplifier for your system.
All high-end microphones, especially those used for sound level meters, require preamps to ensure the highest sound level readings. If you are performing a sound study, you will want the proper preamplifier for your sound level meter.
As with other audio equipment, what you pay for is generally what you get with preamplifiers. If you are a hobbyist just starting out, you are fine to learn the ropes without an external preamp. Most systems have some kind of built-in preamp which will get you started. Once you are ready for more advanced systems, the manufacturers’ websites can be a treasure trove of information to find the preamplifier that is right for you.